Current Digital Subscriber Line (DSL) systems either use Frequency Division Duplexing (FDD), or Echo Cancelled (EC) frequency plans. In the FDD frequency plans, the upstream (from customer premises equipment (CPE) to Central Office (CO) or Remote Terminal (RT)) and downstream signals (from CO or RT to CPE) reside in different portions of the spectrum. Furthermore, in these FDD systems, the lowest portion of the frequency spectrum, where voiceband signals are present, are not used by the DSL modems. This allows the FDD DSL modems to share the line with the voiceband analog voice equipment. The baseband (e.g., systems commonly called SDSL (Symmetric DSL), HDSL2 (High-Speed DSL 2), ISDN (Integrated Services Digital Network), etc.) EC systems use the same frequency bands for both upstream and downstream signals, employing echo cancellers to remove the locally transmitted signal from the received signal. These baseband DSL modems include the lowest frequency portion of the spectrum in their signals, which means that they cannot share a line with voiceband analog voice equipment. One version of the system commonly called Asymmetric Digital Subscriber Line (ADSL) also employs EC, but uses different frequency bands for upstream and downstream traffic. While the downstream band totally overlaps the upstream band, the bands are unequal, with the downstream band extending to greater frequencies than the upstream band. FIG. 1 illustrates some of these band plans.
FIG. 2 shows the loop loss, measured in dB/kft, of 24 AWG twisted pair telephone wire, as a function of frequency. FIG. 3 shows the 99th percentile worst case near end (NEXT) crosstalk coupling gain, as a function of frequency. As shown in FIGS. 2 and 3, the loop loss increases and the amount of crosstalk coupling increases as a function of frequency. This means that successful communication becomes more difficult as the frequency of the signal increases.
Optical Ethernet provides a universal medium for all applications, with the flexibility and bandwidth to support future application demands as well. But unlike the copper infrastructure, which is deployed to every home and building, the existing fiber infrastructure is limited in the number of locations directly connected to the fiber. Trenching fiber to new locations not only impacts the capital costs, but also severely limits the velocity of deployment. As a result, the Optical Ethernet opportunity is limited. While a service provider may deploy a Metro Area Network (MAN) using Optical Ethernet, most customers, with their Ethernet LANs at the ready, are left waiting.
The existing copper infrastructure, with its universal penetration, offers an opportunity to close the chasm between the MAN and the LAN. Conventional xDSL technologies, however, suffer from their own problems. Most important, they are based on ATM, making them a strange partner in an end-to-end Ethernet network. In addition, their deployment velocity is limited by the requirement that service providers condition their copper loops to bring them up to a quality that can support the technology. Conventional xDSL technologies also suffer from spectral compatibility issues, impacting reliability and complicating deployment, as service providers manage their binders to ensure that incompatible technologies are kept away from each other. Also, these technologies are fixed in symmetry, forcing the service provider to mix and match technologies to applications, and deal with the inevitable complexity. Finally, these technologies are limited in reach, typically to 18,000 feet but often derated to 15,000 feet in order to avoid problems.
Ethernet over VDSL (EoVDSL) uses the VDSL band plan to transmit Ethernet frames. As an Ethernet-based technology, it fits within an end-to-end Ethernet strategy. But EoVDSL is limited in reach to 4,500 feet, making it suited more for in-building applications. As a variant of VDSL, EoVDSL suffers from many of the same issues as conventional xDSL, such as spectral compatibility concerns, low deployment velocity, and fixed bandwidth symmetry.